A growing concern related to wind turbines is acoustic emission, in particular emission of noise being also referred to as noise pollution or noise emission. Thereby, noise caused by incoming or inflowing wind, in particular caused by inflow wind turbulence hitting the wind turbine has long been suspected as a substantial contributor to wind turbine noise emissions. Especially high levels of wind turbulence result in increased acoustic emission. To compensate that, a wind turbine may be curtailed for high levels of turbulence in order to reduce the acoustic emission.
One possibility for determining turbulence characteristics like, e.g., measuring levels of turbulence (which may be also referred to as “turbulence intensity”) of inflow wind has been based on cup anemometer wind speed measurements. Cup anemometers require averaging times on the order of, e.g., 10 minutes for measuring turbulence intensity and are therefore unsuitable for an accurate turbine control considering turbulence intensity.
According to one possible embodiment, one or more anemometers may be mounted on a meteorological tower (“MET tower”) which may be located, e.g., approximately 200 m towards a predominant wind direction. Different types of anemometers may be used like, e.g., cup anemometer/wind vane combinations located at the tower at various heights. As an advantageous embodiment ultrasonic anemometers may be used comprising no moving parts and applying a sample rate of, e.g., 20 Hz, making them particularly suited for accurately resolving turbulence characteristics.
The distance between the wind turbine and the MET-tower may present some uncertainty in the resulting turbulence characteristics at a rotor plane of the wind turbine. That is the turbulence is measured 200 m upwind of the wind turbine and will evolve reaching the wind turbine.